Selenium (Se) and tellurium (Te) are oxygen group elements that share chemical properties with sulfur, but also show subtle differences that are useful to constrain magmas genesis and source. Using high-precision isotope dilution measurements, this study characterizes the behavior of Se and Te, relatively to S, during the degassing and crystallization of cogenetic magmas from the southwest Indian Ocean. The study of subaerial and submarine lavas from La Réunion Island first shows that, unlike S and Te, Se is not significantly lost during the rapid, final ascent of less differentiated magmas (MgO > 6wt%). This is because Se is retained as selenide in primary melts despite relatively oxidized conditions. Conversely, selenium degassing occurs during magma crystallization below 6 wt% MgO, at a similar rate as sulfur, yielding small S/Se fractionation in the residual magma. Sulfide fractionation, which begins between 5 and 4 wt% MgO, has minor effect on Se compared to Cu. Tellurium is extensively outgassed from La Réunion subaerial lavas but shows a very specific behavior in submarine lavas where it is retained and even accumulate near 6 wt% MgO. The very different behaviors of Se and Te during magma degassing yields extremely large Se/Te variations between subaerial (up to 225) and submarine (down to 1) lavas from La Réunion. The Fani Maoré eruption that occurred between 2018 and 2021 offshore the island of Mayotte (Comoros Archipelago) also provided the opportunity to study chalcogens degassing during the emplacement of submarine lavas. The study of a pillow lava showing a radial textural zonation typical of degassing-crystallization during cooling reveals that S, Se and other moderately volatile elements (Mo, W, Cs) are the most depleted at intermediate depth within the pillow where pipe vesicles occur, suggesting these elements escape with major volatiles. Conversely, Te is depleted in the whole interior of the pillow, suggesting Te is more efficiently lost due to a lower solubility in magma. Assessment of pillow degassing during emplacement yields a 6.5% loss of the initial S content, 7.2% for Te, but only 1.6% for Se confirming the lower volatility of Se during lava emplacement. Chalcogens degassing is accompanied with significant loss of W (8.9%), Mo (5.1%) and Cs (5.1%) from Fani Maoré lavas. Based on these values, the 6.5 km3 Fani Maoré eruption released 1.1 – 1.5 x 106 tons of S, 2300 – 2700 tons of Mo, 970 – 1130 tons of W, 194 – 339 tons of Cs, 13.3 – 16 tons of Se and 4.5 – 11.8 tons of Te during emplacement. The unusually elevated S/Se ratios (16929 — 22130) of Fani Maoré lavas requires a mantle source extensively fertilized by sulfide melts, whereas the reconstructed S – Se – Te composition of La Réunion plume source is consistent with the preservation of chondritic S/Se and Se/Te ratios.